Determination of oil and water compositions of oil/water emulsions using low field NMR Relaxometry

ABSTRACT

A method and apparatus for determining the oil and water content of a heavy oil and water emulsion includes a low field NMR spectrometer and means for determining the total amplitude of an NMR spectrum at specified T 2  values.

PRIORITY CLAIM

[0001] This application claims the priority benefit of Canadian PatentApplication No. 2,342,007 filed on Mar. 26, 2001 as file no. 45074.9 andentitled Determination of Oil and Water Compositions of Oil/WaterEmulsions Using Low Field NMR Relaxometry.

FIELD OF THE INVENTION

[0002] The present invention relates to methods and apparatuses fordetermining oil and water compositions of heavy oil/water emulsionsusing low field NMR relaxometry.

BACKGROUND OF THE INVENTION

[0003] Low field Nuclear Magnetic Resonance (NMR) relaxometry techniqueshave been developed in the laboratory to enhance and support comparableNMR logging tools that are currently used downhole. Low field NMRrelaxometry has shown that discrimination of water and oil saturation incore and ore can be easily determined. In such cases the NMR can detectthe total water weight fraction and the total oil weight fraction, theviscosity of the oil, the amount of bound or mobile water and the amountof mobile or bound oil.

[0004] One particular problem is the determination of oil and watercontent of specific hydrocarbon streams. Of particular interest are thestreams that contain heavy oil in emulsified fluids (water-in-oil oroil-in-water emulsions) which are currently very common in thermalproduction operations and are very difficult to handle. Test separatorsare currently used as the standard way of measuring the flow ofthermally produced wells such as cyclic steam stimulation (CSS), steamassisted gravity drainage (SAGD) and steam flooding wells. The testseparators are inherently incapable of measuring emulsified flow. Otherprobe-type devices suffer from inaccuracies related to the presence ofsolids or gas, salinity, temperature, velocity, emulsion type, and rangeof cut.

[0005] Therefore, there is a need in the art for methods and apparatusesto discriminate quickly, accurately and precisely the amount of heavyoil or bitumen and water in an emulsified fluid stream.

SUMMARY OF THE INVENTION

[0006] The present invention is based on the discovery that the NMRspectra of an emulsified mixture of heavy oil or bitumen and waterconsists of two sets of T₂ relaxation peaks. At the specific temperatureof 30° C., the water peaks are typically in the range of 10 to 3000milliseconds while the oil/bitumen peaks are typically in the range of0.2 to 10.0 milliseconds. The ranges of these peaks may be affected bythe degree of emulsification or separation of the hydrocarbon andaqueous phases, the temperature and the presence of additives. Thespectrum of the oil/bitumen component diminishes at lower temperaturesand may not be completely recovered at relatively lower temperatures.

[0007] Therefore, in one aspect of the invention, there is provided amethod of determining the oil content of a fluid emulsion comprisingheavy oil and water comprising the steps of:

[0008] (a) providing a low field NMR relaxometer;

[0009] (b) measuring and recording the T₂ relaxation spectrum of theemulsion at a temperature allowing recovery of the T₂ spectrum of theheavy oil;

[0010] (c) determining a T₂ cutoff value;

[0011] (d) measuring the total amplitude of the spectrum at T₂ timesless than and equal to the T₂ cutoff value (A_(oil)); and

[0012] (e) converting A_(oil) to a weight value by dividing A_(oil) bythe amplitude index of an oil standard of known weight (AI_(oil)).

[0013] In another aspect, the invention comprises a method ofdetermining the water content of a fluid emulsion comprising heavy oiland water comprising the steps of:

[0014] (a) providing a low field NMR relaxometer;

[0015] (b) measuring and recording the T₂ relaxation spectrum of theemulsion;

[0016] (c) determining a T₂ cutoff value;

[0017] (d) measuring the total amplitude of the spectrum at T₂ timesgreater than the T₂ cutoff value (A_(w)); and

[0018] (e) converting A_(w) to a weight value by dividing A_(w) by theamplitude index of a water standard of known weight (AI_(w)).

[0019] In another aspect, the invention comprises an apparatus fordetermining the oil content of a flowing fluid emulsion comprising heavyoil and water comprising:

[0020] (a) a low field NMR relaxometer having a NMR magnet positioned inproximity to a channel through which the emulsion flows, saidrelaxometer for measuring the T₂ spectrum of a the sample;

[0021] (b) means connected to the relaxometer for measuring total T₂amplitude below a T₂ cutoff value, wherein a substantial portion of thespectrum attributable to the oil is at T₂ values less than or equal tothe T₂ cutoff value; and

[0022] (c) means for converting the total T₂ amplitude value to a weightvalue.

[0023] In yet another aspect, the invention comprises an apparatus fordetermining the oil content of a fluid emulsion comprising heavy oil andwater comprising:

[0024] (a) means for obtaining a sample of the emulsion;

[0025] (b) a low field NMR relaxometer for measuring the T₂ spectrum ofthe sample;

[0026] (c) means connected to the NMR relaxometer for measuring total T₂amplitude below a T₂ cutoff value, wherein a substantial portion of thespectrum attributable to the oil is at T₂ values less than or equal tothe T₂ cutoff value; and

[0027] (d) means for converting the total T₂ amplitude value to a weightvalue.

[0028] In another aspect, the invention comprises a method ofdetermining the oil content and water content of a fluid emulsioncomprising heavy oil and water comprising the steps of:

[0029] (a) providing a low field NMR relaxometer;

[0030] (b) measuring and recording the T₂ relaxation spectrum of theemulsion at a temperature allowing recovery of the T₂ spectrum of theheavy oil;

[0031] (c) determining a T₂ cutoff value;

[0032] (d) measuring the total amplitude of the spectrum at T₂ timesless than and equal to the T₂ cutoff value (A_(oil));

[0033] (e) converting A_(oil) to a weight value by dividing A_(oil) bythe amplitude index of an oil standard of known weight (AI_(oil));

[0034] (f) measuring the total amplitude of the spectrum at T₂ timesgreater than the T₂ cutoff value (A_(w)); and

[0035] (g) converting A_(W) to a weight value by dividing A_(W) by theamplitude index of a water standard of known weight (AI_(w)).

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The invention will now be described by way of exemplaryembodiments with reference to the accompanying drawings. In thedrawings:

[0037]FIG. 1 shows a typical NMR T₂ spectra from two differentemulsions.

[0038]FIG. 2 shows the comparison of NMR predicted water content vs.Dean-Stark measured water content for three different batches of samplesform reservoir 1.

[0039]FIG. 3 shows the same results as FIG. 2 but are grouped and thetrend-line is plotted.

[0040]FIG. 4 shows the comparison of the NMR predicted data and theDean-Stark measurement data for three samples of reservoir 2.

[0041]FIG. 5 shows a comparison of the results of reservoir 1 andreservoir 2.

[0042]FIG. 6 shows the same results as Figure but are grouped and thecommon trend-line is plotted.

[0043]FIG. 7 is a schematic representation of one embodiment of theapparatus of the present invention.

[0044]FIG. 8 is a schematic representation of an alternative embodimentof the apparatus of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0045] The present invention provides for a method and apparatus fordetermining the oil content or water content, or both oil content andwater content, of a fluid emulsion comprising heavy oil and water. Theinvention is equally applicable to oil-in-water emulsions orwater-in-oil emulsions, whether or not solids, gases or other impuritiesare present and regardless of the range of oil or water content in theemulsion.

[0046] When describing the present invention, the following terms havethe following meanings, unless indicated otherwise. All terms notdefined herein have their common art-recognized meanings.

[0047] A. Definitions

[0048] “AI” or “amplitude index” refers to the amplitude of NMR signalfor one gram of oil or water at surface temperature and atmosphericpressure.

[0049] “Bitumen” refers to hydrocarbon crude oil normally ofviscosity >100,000 cp at room temperature and/or API gravity <10.

[0050] “CPMG” refers to the Curr-Purcell-Meiboom-Gill pulse echo methodused by NMR tools to measure T₂.

[0051] “Emulsion” refers to mixtures of oil and water where one liquidis dispersed in the other liquid as discrete droplets. As used herein,the term “emulsion” may also refer to mixtures of oil and water whereonly a portion of the mixture is an emulsion and the remainder comprisesoil and water as separate phases.

[0052] “Heavy oil” refers to hydrocarbon crude oil normally ofviscosity >20 cp at room temperature and/or API gravity >10 and <20.

[0053] “HI” or “hydrogen index” refers to the relative proton density ofa sample. The strength or amplitude of a signal is thereforeproportional to the the amount of hydrogen in the sample. The HI of purewater at surface temperature and pressure is 1. The HI is proportionalto water concentration found in the Concentrative Properties of AqueousSolutions table for sodium chloride in the CRC Handbook of Chemistry andPhysics (1982). As used herein, HI is used interchangeably with AIabove.

[0054] “NMR” refers to Nuclear Magnetic Resonance which is thetechnology that uses a magnetic field to influence and measure nucleispins of certain elements.

[0055] “TE” refers to Time Echo [ms]. This is the time-to-echo time. Itis defined as the “delay” between pulses.

[0056] “T₂” refers to the transverse relaxation time measured inmilliseconds.

[0057] B. Description

[0058] All NMR measurements were performed using a Numar Corespec1000™relaxometer. Equivalent or alternative relaxometers are well-known inthe art. T₂ measurements were made using the CPMG techniques which arewell known in the art. The NMR magnet set-up was at 30° C. Therelaxometer was tuned twice per day and tuning was done using a sealedstandard sample (permanent amount of doped water with T₂˜240 ms) and astandard tuning procedure.

[0059] The NMR spectra of a mixture of heavy oil or bitumen with waterconsist of two sets of T₂ relaxation peaks. The water peaks aretypically in the range of 10-3000 ms, while the oil/bitumen peaks aretypically in the range of 0.2-10 ms at the specific temperature of 30°C. The T₂ cutoff value is that value which substantially separates thewater peaks from the oil/bitumen peaks. In this case, the T₂ cutoffvalue is about 10 milliseconds. The appropriate cutoff value value forany given application may be determined empirically. Alternatively, avariable such as process temperature may be varied to achieve a desiredcutoff value value. The degree of emulsification or separation of thephases, in addition to the temperature and the presence of additivesaffects the ranges of these peaks. If the spectrum of a mixture is takenat a relatively low temperature, then the complete spectrum of theoil/bitumen may not be recovered.

[0060] If a known amount of a heavy oil (or bitumen) and water mixtureis placed in the NMR, then the spectrum obtained can be deciphered inthe spectra of the individual phases. The total obtained signalamplitude is compared to that of the same amount of a sample that onlycontains water (standard). If the amplitude of the water standard isA_(sw) and the amplitude of the water content of the unknown sample isA_(w) then the water fraction W_(w) of the unknown sample is

W _(w) =A _(w) /AI _(w)

A _(w) =ΣA _(j) ,j>10 ms

[0061] where AI_(w) is the amplitude index of water standard of weightW_(sw)(AI_(w)=A_(sw)/W_(sw)) and ΣA_(j),j>10 ms is the sum of theamplitudes of the sample spectrum above the T₂ cutoff value of 10 ms.The amount of heavy oil or bitumen W_(oil) may then be determined bydifference if there is not a significant amount of solids or gas in thesample:

W _(oil)=1−W _(w)

[0062] This is the simplest way to calculate water and heavy oil contentin any unknown sample of a known weight.

[0063] If the whole spectrum of the liquids is obtained, then the heavyoil fraction W_(oil) can be obtained through a similar equation as thewater content as long as the amplitude of the heavy oil spectrum A_(oil)is corrected for its Amplitude Index (AI_(oil)) at a given temperature.The following equations apply:

W _(oil) =A _(oil) /AI _(oil)

A _(oil) =ΣA _(j) ,j≦10 ms

[0064] where AI_(oil) is the amplitude index of a bitumen standard ofweight W_(s oil)(AI_(oil)=A_(s oil)/W_(s oil)) and ΣA_(j),j≦10 ms is thesum of the amplitudes of the sample spectrum below or equal to the T₂cutoff value of 10 ms. This second equation is valuable when the weightof the sample is not known or there is suspicion of solids or gaspresent in the stream.

[0065] The fact that oil and water contents are estimated from theindividual spectra components provides a set of measurements that areindependent of the presence of gas or solids. This is because both gasand solids such as entrained sand do not contribute to the measuredspectra.

[0066] However, when these types of measurements are attempted in thepresence of solids and/or gas, it is important to capture the completespectrum of the oil. This is achieved by adjusting the temperature atwhich the measurements are taken. In general, an optimum temperature canbe found in the range of 20-80° C. that can be tailored for applicationin specific reservoirs.

[0067] The present invention also relates to an apparatus for performingthe operations disclosed herein. In one embodiment, the inventioncomprises a NMR based system for direct measurement of water and oilfractions. The system is designed to operate using slipstreams for themeasurement of spectra of flowing streams on-line. In one embodiment,the system allows for automatic sampling from a flowing stream and themeasurement is taken from the discrete sample. One embodiment of such asystem is shown schematically in FIG. 7. A NMR magnet (12) is placedadjacent the sample chamber (14) of known volume. The sample is takenfrom the fluid stream by a tap (16) including valve (18). The NMRspectrometer (20) is operatively connected to a processor (22) which maybe a general purpose computer programmed with appropriate software. Theprocessor (22) comprises the means for implementing the methodsdisclosed herein with the NMR data received from the spectrometer (20)and also controls the spectrometer (20). Alternatively, the processor(22) may comprise programmable firmware, electronic circuits or otherhardware, or combination of hardware and software, known to thoseskilled in the art. The system may be controlled by a programmed logiccontroller (24) as is well known in the art.

[0068] Alternatively, the system may be configured to measure a flowingstream without the need to capture a sample and hold it. FIG. 8illustrates schematically one embodiment of a system configured tomeasure a flowing stream.

[0069] As shown in FIG. 8, a NMR magnet (12) is placed directly adjacentthe fluid stream. The NMR spectrometer (20) is operatively connected toa processor (22). The processor (22) comprises the means forimplementing the methods disclosed herein with the NMR data receivedfrom the spectrometer (20) and also controls the spectrometer (20). Thesystem may be controlled by a programmed logic controller (24) as iswell known in the art.

[0070] In either embodiment, the sample pipe need not be full foraccurate measurements to be made. The water content and the heavy oilcontent are measured independently and the ratio determined. If thesample pipe is full with water and oil, and it is known that there areno appreciable quantities of gas or solids, then one of the water or oilphases may be measured and the other determined by difference. However,this approach is potentially more open to error caused by the presenceof gas and solids in the sample stream.

[0071] C. Example

[0072] The capability of the present invention was demonstrated usingsamples from two different heavy oil reservoirs in Western Canada. Twodifferent procedures were tested. For reservoir 1, a variety of sampleswere prepared in the laboratory. The samples had water contents thatcovered the full range. The samples were treated as “blinds”. NMRtesting was done and the water content was calculated for all samples.The samples were then placed in the Dean Stark apparatus and the watercontent was determined. For reservoir 2, samples were obtained from thewellhead and they were brought in the laboratory for water content andoil content determination. First NMR testing was done followed byDean-Stark measurements to verify the NMR results.

[0073]FIG. 1 shows typical spectra from one sample from each reservoir.In both samples it can be seen that the spectra is split in two partswith a cut-off point of approximately 10 ms. The spectra are thenmanipulated as discussed above to provide the water and oil contents.

[0074]FIG. 2 shows the comparison of NMR predicted water content vs.Dean-Stark measured water content for three different batches of samplesfrom reservoir 1. Although the batches were prepared at different timesand were tested by different people the results fall in the same line.In FIG. 3 the same results are grouped and the trend-line is plotted.The correlation is excellent (0.996).

[0075]FIG. 4 shows the comparison of the NMR predicted data and theDean-Stark measurement data for the samples of reservoir 2. Thecorrelation is also excellent (0.966). FIG. 5 compares the results ofreservoir 1 and reservoir 2. In FIG. 6 the same results are grouped andthe common trend-line is drawn. The overall correlation is 0.992 and itis deemed excellent.

[0076] As will be apparent to those skilled in the art, variousmodifications, adaptations and variations of the foregoing specificdisclosure can be made without departing from the scope of the inventionclaimed herein.

What is claimed is:
 1. A method of determining the oil content of afluid emulsion comprising heavy oil and water comprising the steps of:(a) providing a low field NMR relaxometer; (b) measuring and recordingthe T₂ relaxation spectrum of the emulsion at a temperature allowingrecovery of the T₂ spectrum of the heavy oil; (c) determining a T₂cutoff value; (d) measuring the total amplitude of the spectrum at T₂times less than and equal to the T₂ cutoff value (A_(oil)); and (e)converting A_(oil) to a weight value by dividing A_(oil) by theamplitude index of an oil standard of known weight (AI_(oil)).
 2. Themethod of claim 1 wherein the temperature is about 30° C. and the T₂cutoff value is about 10 milliseconds.
 3. A method of determining thewater content of a fluid emulsion comprising heavy oil and watercomprising the steps of: (a) providing a low field NMR relaxometer; (b)measuring and recording the T₂ relaxation spectrum of the emulsion; (c)determining a T₂ cutoff value; (d) measuring the total amplitude of thespectrum at T₂ times greater than the T₂ cutoff value (A_(w)); and (e)converting A_(w) to a weight value by dividing A_(w) by the amplitudeindex of a water standard of known weight (AI_(w)).
 4. The method ofclaim 3 further comprising the steps of determining the total weight ofthe sample and determining the oil content of the emulsion bysubtracting the water content of the sample from the total weight of thesample.
 5. An apparatus for determining the oil content of a flowingfluid emulsion comprising heavy oil and water comprising: (a) a lowfield NMR relaxometer having a NMR magnet positioned in proximity to achannel through which the emulsion flows, said relaxometer for measuringthe T₂ spectrum of a the sample; (b) means connected to the relaxometerfor measuring total T₂ amplitude below a T₂ cutoff value value, whereina substantial portion of the spectrum attributable to the oil is at T₂values less than or equal to the T₂ cutoff value; and (c) means forconverting the total T₂ amplitude value to a weight value.
 6. Theapparatus of claim 5 wherein the T₂ cutoff value value is about 10milliseconds.
 7. The apparatus of claim 5 wherein the relaxometeroperates at less than about 2 MHz.
 8. The apparatus of claim 7 whereinthe relaxometer operates at about 1 MHz.
 9. The apparatus of claim 5further comprising a heater for heating the emulsion flow.
 10. Anapparatus for determining the oil content of a fluid emulsion comprisingheavy oil and water comprising: (a) means for obtaining a sample of theemulsion; (b) a low field NMR relaxometer for measuring the T₂ spectrumof the sample; (c) means connected to the NMR relaxometer for measuringtotal T₂ amplitude below a T₂ cutoff value, wherein a substantialportion of the spectrum attributable to the oil is at T₂ values lessthan or equal to the T₂ cutoff value; (d) means for converting the totalT₂ amplitude value to a weight value.
 11. A method of determining theoil content and water content of a fluid emulsion comprising heavy oiland water comprising the steps of: (a) providing a low field NMRrelaxometer; (b) measuring and recording the T₂ relaxation spectrum ofthe emulsion at a temperature allowing recovery of the T₂ spectrum ofthe heavy oil; (c) determining a T₂ cutoff value; (d) measuring thetotal amplitude of the spectrum at T₂ times less than and equal to theT₂ cutoff value (A_(oil)); (e) converting A_(oil) to a weight value bydividing A_(oil) by the amplitude index of an oil standard of knownweight (AI_(oil)); (f) measuring the total amplitude of the spectrum atT₂ times greater than the T₂ cutoff value (A_(w)); and (g) convertingA_(w) to a weight value by dividing A_(w) by the amplitude index of awater standard of known weight (AI_(w)).